Nowadays, many efforts have been conducted trying to develop an optical information recording medium of postscript type which is designed to use a blue laser beam having a wavelength in the vicinity of 350-500 nm (for example, around 405 nm) which is shorter in wavelength than that conventionally employed. In this optical information recording medium, an organic dye compound is employed for creating an optical recording layer. As the organic dye compound absorbs laser beam, the organic dye compound is decomposed (e.g. thermal decomposition) or denatured, thereby bringing about a change in optical properties of the laser beam of recording or regenerating wavelength. This change is then picked up respectively as a modulation factor, thereby making it possible to perform the recording as well as the regeneration.
In conformity with recent trend to increase the density and the velocity of recording, a laser beam of shorter wavelength zone is now increasingly employed in the recording. As the laser beam to be employed is getting shorter in wavelength, the optical recording layer is required to be formed increasingly thinner. Therefore, the development of the optical information recording medium is now being advanced especially attaching importance to the selection of dyes having a higher refractive index.
Namely, it has been practiced to secure the modulation factor by taking advantage of the optical phase difference that can be generated due to a change in refractive index at the recording or regenerating wavelength.
A general structure of an optical information recording medium 1 which is capable of recording and regenerating information by means of a blue laser beam will be explained with reference to FIG. 15.
FIG. 15 illustrates an enlarged cross-sectional view of a main portion of a disc-like optical information recording medium 1. More specifically, FIG. 15 is a cross-sectional view of the optical information recording medium 1 as it is sectioned diametrally. Namely, FIG. 15 schematically shows a cross-section of the optical information recording medium 1 as it is sectioned perpendicular to the surface provided with a pre-groove 7 and also perpendicular to the direction of the pre-groove 7. This optical information recording medium 1 comprises a light-transmitting substrate 2, an optical recording layer 3 (light-absorptive layer) formed on the substrate 2, a light-reflecting layer 4 formed on the optical recording layer 3, and a protective layer 5 (adhesive layer) formed on the light-reflecting layer 4. By the way, under some circumstances, a dummy layer 6 having a predetermined thickness may be laminated on the top of the protective layer 5 so as to make the optical information recording medium 1 have a predetermined thickness which is required by the specification.
The substrate 2 is provided in advance with a pre-groove 7 which is formed spirally. On both sides of this pre-groove 7, there are located lands 8 constituting the regions other than the pre-groove 7.
As shown in FIG. 15, as a laser beam (recording beam) 9 is irradiated onto the optical information recording medium 1 from the light-transmitting substrate 2 (incident layer) side, the optical recording layer 3 is caused to generate heat (or absorb heat) as the energy of the laser beam 9 is absorbed by the optical recording layer 3, thereby forming a recording pit 10 through the thermal decomposition of the optical recording layer 3.
By the way, the substrate 2 is contacted, through a first boundary layer (or interface) 11, with the optical recording layer 3.
The optical recording layer 3 is contacted, through a second boundary layer 12, with the light-reflecting layer 4.
The light-reflecting layer 4 is contacted, through a third boundary layer 13, with the protective layer 5.
The protective layer 5 is contacted, through a fourth boundary layer 14, with the dummy substrate 6.
The light-transmitting substrate 2 can be generally formed using a resin having a high transparency exhibiting a refractive index ranging from about 1.5 to 1.7 to a laser beam and being excellent in impact resistance. For example, the light-transmitting substrate 2 can be formed using a polycarbonate plate, an acrylic plate, an epoxy resin plate, etc. It is also possible to use a glass plate as the light-transmitting substrate 2.
The optical recording layer 3 deposited on the substrate 2 is formed of a layer made of a light-absorptive substance (a light-absorbing substance) containing a dyestuff. This optical recording layer 3 is caused to generate heat as it is irradiated with a laser beam 9, thereby bring about the thermal decomposition, the heat generation, the absorption of heat, melting, sublimation, deformation or denaturing. This optical recording layer 3 can be formed, for example, by uniformly coating an azo-based dye, a cyanine dye, etc., which has been dissolved in a solvent, on the surface of substrate 2 by means of spin-coating method, etc.
With respect to the materials to be employed for forming the optical recording layer 3, although it is possible to employ any kind of optical recording materials, it is more preferable to employ a photoabsorptive organic dye.
The light-reflecting layer 4 is formed of a metal film which is high in heat conductivity and light reflectance, and can be created by the deposition of gold, silver, copper, aluminum, or an alloy comprising any of these metals by means of vapor deposition method, sputtering method, etc.
The protective layer 5 can be formed, as in the case of the substrate 2, by making use of a resin which is excellent in impact resistance and adhesion. For example, the protective layer 5 can be formed by coating an ultraviolet-curing resin on the light-reflecting layer 4 by means of spin-coating method followed by the curing of the coated layer through the irradiation of ultraviolet rays thereto.
The dummy layer 6 can be formed by making use of the same kinds of materials as those of the substrate 2, thereby securing a predetermined thickness (about 1.2 mm) of the optical information recording medium.
Further, FIG. 16 is an enlarged cross-sectional view illustrating, as in the case of FIG. 15, a main portion of a disc-like optical information recording medium 20 of another type using a blue laser beam. In this case, the optical information recording medium 20 comprises a light-transmitting substrate 2 having a thickness of 1.1 mm, a light-reflecting layer 4 formed on the substrate 2, an optical recording layer 3 (light-absorptive layer) formed on the light-reflecting layer 4, a protective layer 5 formed on the optical recording layer 3, an adhesive layer 21 formed on the protective layer 5, and a cover layer 22 having a thickness of 0.1 mm and formed on the adhesive layer 21.
The substrate 2 is provided in advance with a pre-groove 7 which is formed spirally. On both sides of this pre-groove 7, there are located lands 8 constituting the regions other than the pre-groove 7.
By the way, if a boundary layer between the substrate 2 and the optical recording layer 3 satisfies a low reflectance and, furthermore, if the light reflected from the optical recording layer 3 is sufficient enough, the provision of light-reflecting layer 4 may not be required.
As shown in FIG. 16, as a laser beam (recording beam) 9 is irradiated onto the optical information recording medium 20 from the light-transmitting incident layer (the cover layer 22) side, the optical recording layer 3 is caused to generate heat (or absorb heat) as the energy of the laser beam 9 is absorbed by the optical recording layer 3, thereby forming a recording pit 10 through the thermal decomposition of the optical recording layer 3.
By the way, the substrate 2 is contacted, through a first boundary layer 23, with the light-reflecting layer 4.
The light-reflecting layer 4 is contacted, through a second boundary layer 24, with the optical recording layer 3.
The optical recording layer 3 is contacted, through a third boundary layer 25, with the protective layer 5.
The protective layer 5 is contacted, through a fourth boundary layer 26, with the adhesive layer 21.
The adhesive layer 21 is contacted, through a fifth boundary layer 27, with the cover layer 22.
In the optical information recording medium 1 or the optical information recording medium 20, which are constructed as described above, the laser beam 9 is irradiated onto the recording film of the optical recording layer 3 through the transparent substrate 2 or through the cover layer 22, and the energy of this beam is converted into a thermal energy, thereby bring about the generation of heat causing thermal decomposition, i.e. heating, melting, sublimation, or decomposition, thus creating a recording pit 10. By making use of this recording pit 10, the contrast that can be created due to the reflectance of light at the recorded portion or unrecorded portion is read out as an electric signal (modulation factor).
As described above, in the optical information recording medium 1 or the optical information recording medium 20, the recording pit 10 is created mainly through the change in refractive index that can be caused due to the thermal decomposition of an organic dye compound as the laser beam 9 is irradiated onto the optical recording layer 3 formed of the organic color dye compound.
As a result, the optical constant and decomposition behavior of the organic color dye compound to be employed in the optical recording layer 3 are now important factors in order to create excellent recording pit.
Therefore, the organic color dye compound to be employed in the optical recording layer 3 is now required to be selected from those which are excellent in optical properties and in decomposition behavior to blue laser wavelength. Namely, with the view of securing excellent recording/regenerating properties even in the blue laser wavelength zone, there has been tried to enhance the reflectance of the optical recording layer 3 when it is not yet recorded and to cause a large change in refractive index as the organic dye compound is thermally decomposed by the irradiation of laser beam (thereby making it possible to secure a large modulation factor).
The change of refractive index “n” and of the extinction coefficient “k” of the organic dye compound employed as the optical recording layer 3 due to the absorption of laser beam 9 by the organic dye compound will be explained in reference to FIG. 17.
FIG. 17 is a graph showing the relationship between the wavelength of the laser beam 9 and the refractive index “n” and the relationship between the wavelength of the laser beam 9 and the extinction coefficient “k”. As the organic dye compound employed is thermally decomposed, the molecular bond is destroyed, resulting in the decrease of refractive index “n” after recording and hence resulting in the decrease of reflectance “R”. Therefore, there has been conventionally tried to increase the magnitude of change “Δn” in refractive index “n” to thereby secure a sufficient magnitude of the change “ΔR” in reflectance, thus securing a modulation factor between the recording pit 10 and other portions of the optical recording layer 3 and making it possible to perform regenerable recording.
Further, the graph describing the extinction coefficient “k” is substantially the same as the graph describing the absorbency to the laser beam 9 of the dye, so that it is generally practiced to make the wavelength on the long wavelength side of this absorption peak (recording wavelength) the same as the wavelength of recording beam (laser beam 9). Namely, since the graph of refractive index “n” indicates the absorption peak on the long wavelength side of the graph of the extinction coefficient “k”, it is possible to secure a large magnitude of change “Δn” in refractive index “n” at this recording wavelength.
On the other hand, it is well known that, as in the case of the refractive index “n”, the extinction coefficient “k” is caused to decrease after recording at the same recording wavelength due to the destruction of molecular bond of the organic dye compound as a result of thermal decomposition of the organic dye compound. As the extinction coefficient “k” is decreased, the reflectance “R” is caused to increase. Namely, the change “Δk” of the extinction coefficient “k” acts to decrease the magnitude of change “ΔR” in the reflectance “R” due to the change “Δn” in refractive index “n”. Namely, the absolute value of “ΔR” can be derived as a magnitude corresponding to (Δn−Δk). Therefore, according to the prior art, the interest of development of optical information recording medium is focused to find a way to increase the “Δn” as much as possible in order to secure a desired level of modulation factor and to select a dye exhibiting as small magnitude of “Δk” as possible.
Actually however, no one has succeeded as yet to obtain a suitable dye which is useful as a coloring material of the optical recording layer 3 with respect to the laser beam 9 having a recording wavelength within the range of 350-500 nm (for example, around 405 nm).
As described above, the coloring material is required to exhibit a suitable extinction coefficient “k” (absorption coefficient) and a large refractive index “n” so as to secure a sufficient contrast before and after the recording. Therefore, the coloring material has been selected such that the recording/regenerating wavelength can be located at the skirts of the long wavelength side of absorption peak of absorption spectrum of dye, thus designing the optical recording layer 3 so as to secure a large magnitude of change “Δn” in refractive index “n”.
Further, as for the properties demanded of the organic compound, the behavior of decomposition thereof is required to be suitably selected in addition to the aforementioned optical properties to the blue laser beam wavelength. However, the materials having an optical property indicating a refractive index “n” which is comparable to that of the conventional CD-R or DVD-R in this short wavelength zone is extremely limited in kinds. Namely, in order to bring the absorption band of an organic compound close to the wavelength of blue laser beam, the molecular skeleton of the organic compound is required to be minimized or the conjugated system of the organic compound is required to be shortened. However, when the structure of the organic compound is adjusted in this manner, it will invite the decrease of the extinction coefficient “k” or the decrease of the refractive index “n”.
As for the method of overcoming these problems, there have been recently reported possibilities of improving the optical properties of organic compound through the utilization of interaction between molecules such as association instead of utilizing the optical properties of the simple substance of dye molecule. However, no one has succeeded as yet to realize satisfactory recording properties by making use of such a method.
Meanwhile, if a high-speed recording is to be performed by making use of the optical information recording medium 1 or the optical information recording medium 20, it is required to perform a predetermined recording in a shorter period of time than that required in the conventional speed of recording or low speed recording. Therefore, the recording power is required to be increased, thus increasing a quantity of heat or a quantity of heat per unit time at the optical recording layer 3 on the occasion of the recording. As a result, the problem of thermal strain tends to become more prominent, thus giving rise to the generation of non-uniformity of recording pits 10. Further, since there is a limit in increasing the output power of semiconductor laser for emitting the laser beam 9, it is now demanded to develop a dyestuff having such a high sensitivity that can be coped with a high-speed recording.
As described above, since an organic dye compound is employed in the optical recording layer 3 in the cases of the optical information recording mediums 1 and 20, the development of optical information recording medium is mainly directed to a dyestuff which is capable of exhibiting high refractive index to the laser beam 9 of short wavelength side.
Namely, as long as the conventional postscript type optical information recording medium using an organic dye compound is concerned, only the organic compounds which are capable of exhibiting a large refractive index and a relatively small extinction coefficient (0.05-0.07) to a recording/regenerating wavelength are enabled to be employed in viewpoints of securing modulation factor and reflectance.
Since there are known at present a large number of organic dye compounds having an absorption band which is close to the blue laser wavelength, the extinction coefficient “k” thereof can be controlled. However, since these organic dye compounds fail to have a large refractive index “n”, the dye compound layer is required to have a certain degree of film thickness in order to obtain modulation factor securing a sufficient optical phase difference of the recording portion (recording pit 10).
However, since the organic compound is incapable of exhibiting a large absorption power to the recording beam, it is impossible to make thinner the film thickness of the organic compound film. Therefore, it is required to employ a substrate 2 having a deep trench (since the film of organic compound is formed by means of spin-coating, the increase of film thickness of the organic compound film is achieved by depositing the organic compound in a deep trench). In this case however, it is very difficult to fabricate a substrate 2 having a deep trench, giving rise to the deterioration in quality of the optical information recording medium.
Additionally, since the recording medium where blue laser beam is employed is demanded to execute a high-density recording, the physical track pitch is required to be formed narrow. As a result, when a laser beam of high output such as blue laser beam is employed, the heat generated on the occasion of the decomposition of dye compound is liable to be accumulated, thus enabling the heat generated on the occasion of the decomposition of dye compound to easily transmit to the neighboring tracks and hence raising the problem that the properties of the recording medium will be deteriorated.
As described above, the recording medium using a blue laser wavelength zone where the refractive index of dyestuff (organic dye compound) cannot be easily enhanced is accompanied with the problem that it is difficult to enable the recording medium to exhibit excellent recording properties in the conventional recording to be effected through the phase change of the change of refractive index “n”.
Patent document 1: JP-A 2003-223740 (KOKAI)
Patent document 2: JP-A 2003-331465 (KOKAI)
Patent document 3: JP-A 2004-142131 (KOKAI)
Patent document 4: JP-A 2004-216714 (KOKAI)